The present invention relates to a drive circuit for a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation.
A related drive circuit for a fluid motor is shown in FIG. 10. This drive circuit is comprised of a fluid motor 12 having a swash plate 11 which is tiltable between positions of high-speed and low-speed rotation; a reduction gear 10 coupled to the fluid motor 12 and adapted to reduce the output rotational speed of the fluid motor 12 before outputting it; a tilting piston 13 which, when a high-pressure fluid is introduced to it, pushes and tilts the swash plate 11 to the position of high-speed rotation, and which, when set under a tank pressure, allows the swash plate 11 to tilt to the position of low-speed rotation; a changeover valve 18 which is interposed midway in a connecting passage 17 for connecting the tilting piston 13 and a selector valve 16 for selectively obtaining the high-pressure fluid from main circuits 14 and 15 of the fluid motor 12, and which is changed over between a high-speed position K for introducing to the tilting piston 13 the high-pressure fluid obtained by the selector valve 16 and a low-speed position L for discharging the fluid acting on the tilting piston 13 to a drain passage 22; a first pilot passage 19 for introducing into the changeover valve 18 a constant-pressure pilot fluid for a direction in which the changeover valve 18 is changed over to the high-speed position K; and a second pilot passage 21 for introducing into the changeover valve 18 the high-pressure fluid selectively obtained from the main circuits 14 and 15 by a counterbalance valve 20, for a direction in which the changeover valve 18 is changed over to the low-speed position L.
In the above-described drive circuit for the fluid motor 12, during a normal load, the fluid force based on the constant-pressure pilot fluid in the first pilot passage 19 is higher than the fluid force based on the high-pressure fluid (the high-pressure fluid selectively obtained from the high pressure-side main circuits 14 and 15) in the second pilot passage 21, so that the changeover valve 18 has been changed over to the high-speed position K. Consequently, the high-pressure fluid from the main circuits 14 and 15 obtained by the selector valve 16 causes the tilting piston 13 to project through the connecting passage 17, thereby holding the swash plate 11 at the position of high-speed rotation.
Next, when the load acting on the above-described fluid motor 12 increases, the pressure in the main circuits 14 and 15 on the high-pressure side increases, so that the fluid force based on the high-pressure fluid in the second pilot passage 21 becomes higher than the fluid force based on the constant-pressure pilot fluid in the first pilot passage 19. Consequently, the changeover valve 18 is changed over to the low-speed position L, so that the fluid acting on the tilting piston 13 is discharged to the drain passage 22, and the swash plate 11 is tilted to the position of low-speed rotation while pushing the tilting piston 13. As a result, the fluid motor 12 rotates at low speed with a large torque, and the pressure within the high pressure-side main circuits 14 and 15 declines.
However, with such a drive circuit for a fluid motor, since the fluid acting on the tilting piston 13 is instantly discharged into the drain passage 22 simultaneously as the changeover valve 18 is changed over to the low-speed position L, the swash plate 11 is suddenly tilted to the position of low-speed rotation, and the pressure within the high pressure-side main circuits 14 and 15 declines. Consequently, the output torque of the fluid motor 12 instantly changes from a small torque with high-speed rotation to a large torque with low-speed rotation, with the result that there are problems in that a shock can be imparted to an operator to deteriorate an operational feeling, and a large load can be imparted to the drive circuit itself and shorten its life.
In addition, if the swash plate 11 is suddenly tilted to the position of low-speed rotation and the pressure within the high pressure-side main circuits 14 and 15 declines substantially as described above, the fluid force based on the constant-pressure pilot fluid in the first pilot passage 19 becomes higher than the fluid force based on the high-pressure fluid in the second pilot passage 21, and the changeover valve 18 is changed over again to the high-speed position K. However, when the changeover valve 18 is thus changed over to the high-speed position K, the pressure within the high pressure-side main circuits 14 and 15 rises again, so that the changeover valve 18 is changed over again to the low-speed position L. Hence, there is a problem in that the changeover valve 18 is repeatedly changed over between the high-speed position K and the low-speed position within a short time, i.e., hunting due to pressure fluctuations of the fluid motor 12 can occur.
The object of the invention is to provide a drive circuit for a fluid motor which is capable of improving the operational feeling and prolong the life of the drive circuit while preventing hunting at the time of changeover of the changeover valve to the low-speed position.
The above object can be attained by a drive circuit for a fluid motor including a fluid motor having a swash plate which is tiltable between positions of high-speed and low-speed rotation; a tilting piston which, when a high-pressure fluid is introduced thereto, pushes and tilts the swash plate and tilts it to the position of high-speed rotation, and which, when set under a tank pressure, allows the swash plate to tilt to the position of low-speed rotation; a changeover valve which is interposed midway in a connecting passage for connecting the tilting piston and a selector valve for selectively obtaining the high-pressure fluid from two main circuits of the fluid motor, and which is changed over between a high-speed position for introducing to the tilting piston the high-pressure fluid obtained by the selector valve and a low-speed position for discharging the fluid acting on the tilting piston to a drain passage; a first pilot passage for introducing into the changeover valve a constant-pressure pilot fluid for a direction in which the changeover valve is changed over to the high-speed position; and a second pilot passage for introducing into the changeover valve the high-pressure fluid selectively obtained from the main circuits, for a direction in which the changeover valve is changed over to the low-speed position, the drive circuit for a fluid motor characterized in that:
an additional pilot passage is provided for introducing from the connecting passage between the changeover valve and the tilting piston into the changeover valve an additional pilot fluid for the direction in which the changeover valve is changed over to the low-speed position, and that a first narrow passage for allowing the drain passage and the connecting passage extending from the changeover valve on a tilting piston side to communicate with each other and a second narrow passage for allowing the connecting passage extending from the changeover valve on the tilting piston side and the communicating passage extending from the changeover valve on the selector valve side to communicate with each other are provided midway in a stroke in which the changeover valve is changed over from the high-speed position to the low-speed position.
When the two main circuits are under low pressure and the rotation of the fluid motor is stopped, the constant-pressure pilot fluid is only being introduced into the changeover valve through the first pilot passage, so that the changeover valve remains changed over to the high-speed position.
Next, when the high-pressure fluid is supplied to either one of the main circuits, the fluid motor rotates. At this time, the high-pressure fluid in the high pressure-side main circuit obtained by the selector valve is introduced to the tilting piston through the connecting passage to tilt the swash plate to the position of high-speed rotation. In addition, the high-pressure fluid flowing through this connecting passage is introduced into the changeover valve as an additional pilot fluid through the additional pilot passage.
Consequently, the fluid force based on the constant-pressure pilot fluid in the first pilot passage is applied to the changeover valve as the changing-over force for directing the changeover valve toward the high-speed position. Meanwhile, applied as the changing-over force for directing the changeover valve toward the low-speed position is the resultant force of the fluid force based on the high-pressure fluid obtained from the high pressure-side main circuit and introduced through the second pilot passage and the fluid force based on the additional pilot fluid (under the same pressure as the pressure within the second pilot passage) in the additional pilot passage obtained by the selector valve.
Here, when a normal load is being applied to the fluid motor, the aforementioned changing-over force for directing the changeover valve toward the high-speed position is larger than the aforementioned changing-over force for directing the changeover valve toward the low-speed position, the changeover valve is held at the high-speed position.
Next, when the load acting on the fluid motor increases, the pressure within the high pressure-side main circuit rises, and the resultant force of the fluid force based on the high-pressure fluid in the second pilot passage and the fluid force based on the additional pilot fluid in the additional pilot passage becomes larger than the fluid force based on the constant-pressure pilot fluid in the first pilot passage, so that the changeover valve starts to be changed over from the high-speed position to the low-speed position.
Midway in this process of changeover to the low-speed position, the connecting passage extending from the changeover valve on the tilting piston side communicates with the drain passage through the first narrow passage, so that a small amount of the fluid in the connecting passage at in that region is discharged to the drain passage, and the pressure drops. As a result, the pressing force applied to the swash plate by the tilting piston becomes small, and the swash plate starts to tilt from the position of high-speed rotation toward the position of low-speed rotation.
Here, when the pressure within the connecting passage extending from the changeover valve on the tilting piston side, i.e., the pressure of the additional pilot fluid, drops as described above, the fluid force being applied to the changeover valve by the additional pilot fluid also becomes small, so that the changing-over force (resultant force) for directing the changeover valve toward the low-speed position L becomes small. Further, when this resultant force becomes lower than the fluid force based on the constant-pressure pilot fluid, the changeover valve is pushed back toward the high-speed position, whereas when the resultant force becomes higher than the fluid force based on the constant-pressure pilot fluid, the changeover valve is pushed back toward the low-speed position.
The changeover valve thus moves to the position where a balance is established between the resultant force for directing the changeover valve toward the low-speed position and the fluid force based on the constant-pressure pilot fluid for directing the changeover valve toward the high-speed position. Of this resultant force, the fluid force based on the additional pilot fluid is determined by the amount of fluid flowing out from the connecting passage extending from the changeover valve on the tilting piston side into the drain passage through the first narrow passage and the amount of fluid flowing from connecting passage extending from the changeover valve on the selector valve side into the connecting passage extending from the changeover valve on the tilting piston side through the second narrow passage. The fluid force which is thus imparted to the changeover valve by the additional pilot fluid functions as a pressure regulator for the changeover valve. At this time, the swash plate also tilts to an intermediate position between the position of high-speed rotation and the position of low-speed rotation in correspondence with the pressure within the connecting passage extending from the changeover valve on the tilting piston side.
Then, when the pressure within the high pressure-side main circuit gradually increases in correspondence with the increase in the load acting on the fluid motor, of the resultant force which is balanced with the fluid force of a fixed value based on the constant-pressure pilot fluid, the fluid force based on the high-pressure fluid in the second pilot passage becomes gradually large. Hence, the remaining fluid force, i.e., the fluid force imparted to the changeover valve by the additional pilot fluid, becomes gradually small; namely, the pressure within the connecting passage extending from the changeover valve on the tilting piston side gradually drops, thereby causing the swash plate to tilt gradually toward the position of low-speed rotation. When the pressure within the connecting passage extending from the changeover valve on the tilting piston side is thus caused to drop to the level of the pressure within the drain passage (tank pressure), the changeover valve is changed over to the low-speed position, and the swash plate tilts to the position of low-speed rotation.
While the swash plate tilts from the position of high-speed rotation to the position of low-speed rotation owing to the changeover of the changeover valve to the low-speed position, the pressure within the high pressure-side main circuit of the fluid motor does not undergo a sharp drop and only rises gradually in correspondence with the load under constant-pressure control. Therefore, no shock occurs in the drive circuit, the operational feeling improves, a long life is obtained, and the occurrence of hunting is prevented.
In addition, if the arrangement according to claim 2 is provided, it is possible to control the above-described operation with high accuracy.
Furthermore, if the arrangements according to claims 3 and 5 are provided, the first and second narrow passages can be provided in the spool or the spool chamber of the changeover valve simply and at low cost.
In addition, if the arrangement according to claim 5 is provided, a counterbalance valve, for example, for obtaining from the main circuits the high-pressure fluid (pilot fluid) to be introduced into the changeover valve becomes unnecessary, so that the structure becomes simple, and the fabrication cost can be lowered.
Two or more of the arrangements of claims 2 to 5 can be combined with the arrangement of claim 1 to provided the combined effects.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. 2001-73446 (filed on Mar. 15, 2001) and 2002-5319 (filed on Jan. 11, 2002), which are expressly incorporated herein by reference in their entireties.